Insulating coating compositions and method of making



Patented Dec. 28, 1948 INSULATING COATING COMPOSITIONS AND METHOD OFMAKING John T. Acker, Queens Village, N. .Y.-,,.assignor to BellTelephone Laboratories, Incorporated, New York, N. Y.., a corporation ofNew York Application November 13, 1941, Serial No. 418,894

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This invention relates to coatings for filaments and particularly toinsulating coatings for heater elements of indirectly heated cathodesincorporated in electron discharge devices.

In such devices, for example of the ultra high frequency type, havingdiminutive electrode assemblies to match a definite short wave-lengthrange or of other types of small structure but high heater voltagerating and in which the emitter is of the indirectly heated type, theenergizing element or heater within the emitter is naturally of minutedimensions. This heater element is insulated from the cathode or emitterby a coating of suitable insulating material which is suspended in aliquid in order to .afiix a desirable amount of insulation to the finewire by spraying, dipping or painting the substance thereon.

Various difiiculties have been encountered heretofore, even withinsulating material of the finest particle size obtainable and withconcentrations thereof in various liquids suitable for application bythe usual methods. When the mixture contains a preponderance of solidsthe weight thereof with high viscosity produces undesirable results dueprobably to surface tension effects on the small diameter wire. If theconcentration of solids is decreased the amount of coating for a givennumber of coats is inefficient. Even with a rich mixture, that is, of acreamy paste character, which has been found to serve the purposerequired, the undesirably large number of successive coatings necessaryto attain the required thickness of insulation is ineiiicient, timeconsuming and uneconomical.

An object of this invention is to efliciently apply refractoryinsulating material on very fine wire to produce a uniform, coherent andhighly practical coating thereon.

Another object of the invention relates to increasing both the fluidityof the mixture and the concentration of solids in the mixture to a highdegree.

A further object of the invention is to decrease the number of coatsrequired for a given thickness of insulation.

Another object is to increase the hydrogen ion concentration of thecoating mixture whereby the fluidity of the mixture is materiallyincreased to facilitate the coating of very fine Wire filament bymachine methods.

The coating mixture or suspension contemplated by this inventioninvolves a refractory insulating material, for instance, aluminum oxideof fine particle size, which is mixed with water or any other suitableliquid carrier to form a suspension which may be applied by dipping orpainting the heater Wire. The consistency of the suspension should be acreamy paste for effective results in the coating procedure. The amountof oxide solids capable of being incorporated in the suspension may beincreased more than 5.0. per cent and the fluidity also increased byincreasing the hydrogen ion concentration of the mixture with theaddition of an ionizing medium, such as nitric or hydrochloric acid,without changing the consistency of the suspension.

The hydrogen ion concentration of the mixture imparts high fluidity tothe suspended particles .Of oxide so that they adhere to very fine Wireof the order of .001 inch diameter, and the char? acter of the coatingmixture overcomes inherent surface tension when the wire is travelingthrough the liquid carrier in a machine coating process, whereby thenumber of coatings to secure the desired thickness of insulation ismaterial1 reduced. 1 As a specific example of the coating compositiOn ofthis invention for producing a uniform layer of insulation onvery finetungsten wire of the order of .001 inch diameter, 1000 grams of aluminumoxide of very small particle size, such as 900 mesh, is mixed with 460cubic centimeters of distilled water to form a creamy paste. -By adding,for example, 40 cubic centimeters of nitric or hydrochloric acid to themixture, the fluidity is considerably increased so that 659 grams ofaluminum oxide may be added to the mixture to increase the solid contentapproximately 65 per cent. That is: with approximately less than 10 percent addition of fluid the solids may be increased more than 50 percent.

This is achieved by the action of the nitric acid which increases thehydrogen ion concentration or pH value of the carrier liquid so that theviscosity is materially decreased. The acidulated solution should have ahydrogen ion concentration in the range between .15 and 1.5 normal, theconcentration in the suspension given above being between .77 and .99normal. The composition adheres to the fine wire so that three or fourcoatings or layers are sufiicient to attain the required thickness ofinsulation on the wire, whereas heretofore it was necessary to apply tenor more coatings for the same degree of insulation. The reduced numberof coatings materially avoids lumpy appearing insulation which is theresult of dirt, lint or other foreign matter becoming embedded in theinsulation during the coating process. Furthermore, the suspension isnot sensitive to temperature changes which might affect the solidsubstance being precipitated out of solution nor is the suspensionlimited by hydrolysis since any degree of hydrogen ion concentration maybe obtained by varying the acid quantity in solution.

The acid content is usually mixed with the carrier liquid and the solidcontent added to the consistency of a creamy paste. The specificquantities are not critical so that various proportions may be prepareddepending on the character of coating desired, the particle size andkind of insulating material, and the diameter of the wire to be coated.

The coated wire is fired at a temperature of 1750 C. in hydrogen orother inert atmosphere after each layer is applied, to facilitate thesintera suspension of creamy consistency, a quantity of ing of theinsulation on the wire. This procedure eliminates preliminary baking ata lower temperature prior to the sintering temperature and results inproviding a refractory insulation covering for the wire in which thealuminum oxide particles are tenaciously bonded to the wire without anyreaction products which might contaminate the coating, the water andacid constituents being partially removed by evaporation and completelyremoved during the firing period as diffusible vapors.

While the strong hydrochloric and nitric acids are preferred due totheir high concentration of hydrogen ions, any other suitable acid maybeemployed with satisfactory results for increasing the viscosity of thesuspension, whereby'a definite coating density and thickness may beattained with a lesser number of successive layers.

Although the invention has been disclosed with respect to a definitecoating suspension for application to specific diameter wire for heatertype cathodes, it is, of course, understood that the invention is notlimited to this aspect since various changes may be made in thecharacter and material of the coating solids and solutions or carriersand that other types of wire may be coated to achieve the results ofthis invention. Therefore, the scope of the invention is only to belimited by the appended claims.

What is claimed is: i

1. The method of preparing a coating mixture for wire elements whichcomprises mixing finely divided aluminum oxide and water in proportionstoform a suspension of creamy consistency, adding hydrochloric acid tothe suspension to increase the fluid content by of the order of 10 percent, and adding finely divided aluminum oxide to the suspension toincrease the solid content by of the order of 50 per cent.

substantially 10 per cent by volume of said water of a strong inorganicacid and an additional quantity of finely divided aluminum oxide of theorder of 50 per cent of said first quantity of aluminum oxide, applyingthe resultant mixture to the wireelement in successive coatings, andfiring each coating in an inert atmosphere to bond the oxide to the wireelement and to remove the water and acid.

. 1 JOHN T. ACKER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 424,353 Preussner Mar. 25, 1890449,619 Euricht Mar. 31, 1891 834,633 Neubarger et a1. Oct. 30, 19061,001,413 Keppeler et a1 Aug. 22, 1911 1,376,523 Formby May 3, 19211,409,939 Formby Mar. 21, 1922 1,591,563 Ruff July 6, 1926 1,606,481Rockow Nov. 9, 1926 1,719,988 Myers July 9, 1929 1,877,531 ReichmannSept. 13, 1932 1,908,792 Ruben May 16, 1933 1,934,091 Reichmann Nov. '7,1933 2,041,802 Wilson et al May 26, 1936 2,075,876 Von Wedel Apr. 6,1937 2,097,157. Jameson Oct. 26, 1937 2,115,828 Prescott May 3, 19382,142,331 Prescott -1--- Jan. 3, 1939 2,215,295 Morrill Sept. 17, 19402,292,065 Elsey Aug. 4, 1942 FOREIGN PATENTS I Number Country Date364,529 Great Britain 1931 373,520 Great Britain 1932 OTHER REFERENCESSearle Refractory Materials (1924), pages 96 and 97..

